1. Field of the Invention
The present invention relates to a process for preparing a semiconductor device having a walled emitter structure covering at least one side surface with a dielectric layer for separation of devices.
2. Description of the Prior Art
FIGS. 1(a)-1(f) are sectional views showing steps of preparation of an oxide layer separation type bipolar IC prepared by the conventional process for preparing a semiconductor device. In FIG. 1, the reference (1) designates a p-substrate; (2) designates an embedded collector formed in the p-substrate (1); (3) designates an oxide layer; (4) designates an n-epitaxial layer; (5) designates a separating oxide layer for separation of devices obtained by selective oxidation; (6) designates a field doped layer made of p.sup.+ -layer for prevention of field turn-over formed by implantation of boron ions into a filed part followed by selective oxidation; (7) designates a surface protective oxide layer having relatively thin thickness such as 1000 .ANG.; (8) designates implanted phosphorus ions; (9) designates a resist mask for collector wall; (10) designates a collector wall; (11) designates a resist mask for base formed by photo-engraving; (12) designates implanted boron ions; (13) designates a base; (14a) designates an emitter-opening part; (14b) designates a base-opening part; (14c) designates a collector-opening part; (15) designates a resist mask for emitter; (16) designates implanted arsenic ions; (17) designates an edge of the separating oxide layer; (18) designates a surface protective oxide layer having relatively thick thickness such as 4000 .ANG.; (19) designates an emitter; (20) designates an emitter wiring; (21) designates a base wiring; and (22) designates a collector wiring.
The process for preparing the semiconductor device having the aforementioned structure will be illustrated. As shown in FIG. 1(a), the embedded collector (2) is formed in the p-substrate (1) by thermal diffusion or ion implantation of As or Sb to perform the doping and the elongating diffusion. As shown in FIG. 1(b), the surface oxide layer (3) is removed and n-epitaxial layer (4) is grown and the separating oxide layer (5) for separation of devices is formed by the selective oxidation in the selective oxidation step. Before the selective oxidation, boron ions are implanted into the field part to form the field doped layer (6) made of p.sup.+ -layer for prevention of field turn-over. As shown in FIG. 1(c), the surface protective oxide layer (7) having relatively thin thickness such as 1000 .ANG. is formed and then the opening part of the collector wall is formed by a photo-engraving and the phosphorus ions (8) are implanted with the resist mask (9) for collector wall. As shown in FIG. 1(d), the collector wall (10) is formed. Then, the resist mask (11) for base is formed by a photo-engraving and the boron ions (12) are implanted. As shown in FIG. 1(e), the base (13) is formed. Then, the emitter opening part (14a), the base opening part (14b) and the collector opening part (14c) are opened. The emitter opening part (14a) and the collector opening part (14c) are not covered, but the base opening part (14b) is covered with the resist mask (15) and the emitter and the arsenic ions (16) are implanted. As shown in FIG. 1(f), the emitter (19) is formed. Then, the resist is removed and the emitter wiring (20) and the base wiring (21) and the collector wiring (22) are formed by the metallization. After the step, the passivation or the multilayer wiring step is given by a suitable device.
When the semiconductor device having the walled emitter structure having an emitter whose at least one side surface is covered by a separating oxide layer is prepared, by the conventional process, the structure of the edge (17) of the separating oxide layer (5) and the edge (17) of the emitter opening part (14a) as the mask in the etching of the oxide layer in the formation of the emitter part (14a) and the other opening parts, is different in the implantation of the boron ions (12) for forming the base (13) from the implantation of the arsenic ions (16) for forming the emitter (19). Therefore, as shown in FIG. 2, the base junction is thin in the edge (17) of the separating oxide layer (5) whereas the emitter junction is deep whereby there is disadvantage of easy formation of shortcircuit of emitter-collector caused by passing through the emitter at the part (shown by the circular dotted line in FIG. 2).